Dense wavelength division multiplexing (DWDM) optical communications have been used for optical communication systems using optical fibers. In DWDM optical communications, in order to multiplex light rays emitted from various semiconductor lasers it is necessary to stabilize the wavelengths of those light rays with a high degree of accuracy. Therefore, a wavelength monitoring device that discriminates and monitors the wavelength of a light ray emitted from each semiconductor laser is used. A prior art wavelength monitoring device utilizes the dependence of the intensities of polarized components (an ordinary ray and an extraordinary ray) of light passing through a uniaxial birefringent crystal upon the wavelength of the light. Two uniaxial birefringent crystals that exhibit opposite temperature characteristics against temperature changes are arranged in series in order to provide a temperature compensation function for the wavelength monitoring device.
A brief explanation will be made as to a uniaxial birefringent crystal that is used as a wavelength discriminating filter in the prior art wavelength monitoring device.
A birefringent crystal has optical anisotropy. Particularly, in the case of a uniaxial birefringent crystal, light passing through this crystal is separated into a polarized component (i.e., an ordinary ray) that vibrates in a direction of n0 perpendicular to a plane that consists of an optical axis of the crystal (referred to as C axis from here on) and a laser optical axis of an incident optical signal that is the direction in which the optical signal is traveling and that is determined dependently upon an optical system, and a polarized component (i.e., an extraordinary ray) that vibrates in a direction of ne perpendicular to both the direction of n0 and the laser optical axis. The uniaxial birefringent crystal has different refractive indices for the ordinary ray and the extraordinary ray.
In the prior art wavelength monitoring device, two uniaxial birefringent crystals having opposite temperature characteristics are arranged in series along the direction of the optical axis of incident light so that their crystal cut-out surfaces are perpendicular to the optical axis of the incident light. Each of the two uniaxial birefringent crystals has a C axis that is parallel to the crystal cut-out surface thereof and is inclined at 45 degrees against a y axis, which is a vertical direction, and toward an x axis, which is a horizontal axis, in an xy plane perpendicular to the optical axis of the incident light. The optical signal emitted from the semiconductor laser has a polarized component that vibrates in the direction of the x axis, and passes through the two uniaxial birefringent crystals. In this case, an extraordinary ray vibrates in the direction of the C axis. Because each of the two uniaxial birefringent crystals has different refractive indices for an ordinary ray and an extraordinary ray, a phase difference occurs between the ordinary ray and the extraordinary ray and the intensities of the ordinary ray and the extraordinary ray are determined according to this phase difference. The intensity of a predetermined polarized wave, which is detected by a photodiode of the wavelength monitoring device and is included in the optical signal that originates from the ordinary ray and the extraordinary ray, changes dependently upon the wavelength of the optical signal because the phase difference changes dependently upon the wavelength of the optical signal. Therefore, the prior art wavelength monitoring device in which two uniaxial birefringent crystals having opposite temperature characteristics are arranged in series can monitor the wavelength of an optical signal emitted from the semiconductor laser without being affected by temperature changes of the two uniaxial birefringent crystals.
However, a problem with the prior art wavelength monitoring device constructed as mentioned above is that the two uniaxial birefringent crystals have to be accurately positioned with respect to each other and the optical axis of the incident optical signal, and it is therefore difficult to assemble the prior art wavelength monitoring device.
In the prior art wavelength monitoring device, though the adjustment of the length of each uniaxial birefringent crystal in the direction of the optical axis of the incident optical signal can implement temperature compensation, the temperature compensation cannot be carried out in a wide temperature range. Therefore, another problem is that the prior art wavelength monitoring device cannot carry out temperature compensation in a range of operating temperatures of the semiconductor laser.
A further problem is that when a wavelength range of the optical signal emitted from the semiconductor laser, the center of the wavelength range being a reference wavelength, shifts due to a change in the ambient temperature, for example, the wavelength discriminating characteristic of the wavelength monitoring device degrades and the wavelength monitoring device cannot accurately monitor the wavelength of the optical signal.
The present invention is proposed to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a wavelength monitoring device that can accurately monitor the wavelength of light without being affected by temperature changes and that can be easily assembled.
It is another object of the present invention to provide a wavelength monitoring device that can accurately monitor the wavelength of light in a wide temperature range and without being affected by temperature changes.
It is a further object of the present invention to provide a wavelength monitoring device that can always monitor the wavelength of light with an excellent wavelength discrimination characteristic even if a wavelength range of an optical signal emitted from a semiconductor laser shifts.